The present invention relates to a method and apparatus for recording data on and reproducing data from a hologram recording medium, by utilizing the interference between an object beam and a reference beam.
There has been hitherto proposed a hologram recording/reproducing system. In this system, an object beam modulated in accordance with data to be recorded is made to interfere with a reference beam in a hologram recording medium that acquires great photo-refractive effect. An interference pattern is thereby produced. The interference pattern is recorded as data on the hologram recording medium. A reading beam is applied to the recording medium on which data has been recorded, at the same angle of incidence as the reference beam, thereby reproducing the data from the hologram recording medium.
In the hologram recording/reproducing system, a light beam passes through a spatial light modulator such as a liquid crystal display (LCD) panel. As the light beam passes the modulator, it is modulated in accordance with the data representing the image that is displayed by the spatial light modulator. This light beam is applied, as object beam, into the hologram recording medium. The data representing the image is thereby recorded at once on the hologram recording medium. Data can be reproduced from the medium in units of holograms, each representing an image. Therefore, the data can be accessed at higher speed in this hologram recording/reproducing system than in the data recording/reproducing system which uses an optical disk as recording medium and which is known to achieve relatively high-speed data access.
In this hologram recording/reproducing system, the angle of incidence at which the reference beam is applied to the recording medium can be changed each time a hologram is recorded on the medium. If so, a number of holograms can be recorded, one overlapping another, on the recording medium, thus accomplishing so-called xe2x80x9cmultiple recordingxe2x80x9d. The hologram recording/reproducing system can record data at an extremely high density.
Thus, the hologram recording/reproducing system attracts much attention as a system that satisfies the demands made as the information industry developed in recent years, i.e., the demand for an increase of recording density and the demand for an increase of data-accessing speed.
Generally, hologram recording media are made of photo-refractive crystals such as lithium niobate, which exhibits good optical property and which can form a sufficiently thick layer.
A hologram recording medium made of photo-refractive crystal is, however, disadvantageous. When the medium is exposed to light to reproduce data from the medium, the diffraction efficiency of the hologram recorded in the photo-refractive crystal decreases. As the hologram is repeatedly reproduced from the photo-refractive crystal, the refraction efficiency gradually lowers. Consequently, the S/N ratio of the data reproduced gradually decreases, until at last it becomes impossible to reproduce the hologram from the recording medium.
The diffraction efficiency xcex7writw(t) that a hologram has immediately after it has been recorded in the photo-refractive crystal is given by the following equation (1):
xcex7writw(t)=xcex70xc2x7(1xe2x88x92exe2x88x92xcex1It)xe2x80x83xe2x80x83(1)
where xcex1 is the time constant at the time of recording the hologram, I is the spatial average intensity of light, t is the time that has elapsed from the recording the hologram, and xcex70 is the saturated diffraction efficiency.
The diffraction efficiency xcex7read(t) that a hologram has when it is reproduced from the photo-refractive crystal is given by the following equation(1):
xcex7read(t)=xcex71xc2x7exe2x88x92xcex1xe2x80x2Itxe2x80x83xe2x80x83(2)
where xcex1xe2x80x2 is the time constant at the time of reproducing the hologram, I is the average intensity of light, and xcex71 is the initial diffraction efficiency.
From the equations described above it is understood that the diffraction efficiency of each hologram recorded in the photo-refractive crystal exponentially decreases as the hologram is reproduced. (See P. Heh, Photo-refractive Nonlinear Optics, Maruzen, Tokyo).
As indicated above, the S/N ratio of the data reproduced decreases as the diffraction efficiency of the hologram lowers. If the S/N ratio much lowers at the time of reproducing the data, the signal component will be, so to speak, buried in the noise. This renders it impossible to reproduce the hologram. Thus, to maintain the diffraction efficiency at a value large enough to prolong the hologram, it is desired that some means be provided that would suppress the decrease in the diffraction efficiency, which takes place at the time of reproducing the hologram.
In order to lengthen the lifetime of a hologram, various attempts have been made hitherto. For example, an original hologram recorded in SBN crystal may be copied onto another recording medium made of thermoplastic or the like when the diffraction efficiency lowers as the hologram is reproduced. The hologram thus copied may then recorded again in the SBN crystal in which the original hologram has been recorded. (See D. Brady et al., Opt. Lett. 15,817 (1990).)
To lengthen the lifetime of the hologram by this method, the hologram copied must have higher diffraction efficiency than the original hologram. For this reason, the recording medium, which is used to copy the hologram, is made of thermoplastic, i.e., material exhibiting high diffraction efficiency. Thermoplastic has low resolution, however. A medium made of thermoplastic cannot reliably copy a hologram represented by a great amount of data. Using a plurality of hologram recording media, this method is inevitably complex. To make matters worse, the method requires much time to record and copy a hologram and is therefore not practical.
Another method has been proposed. In this method, a hologram reproduced is read by an imaging device such as a CCD. When the diffraction efficiency of the hologram falls below a threshold value, the image read by the imaging device such as a CCD is displayed by a spatial light modulator such as a liquid crystal display. The image thus displayed is recorded again, thereby refreshing the hologram. (See J. J. P. Drolet et al., Opt. Lett. 22. 552 (1997).)
A hologram recording/reproducing system employing this method has been proposed. In the system, the beam conjugate to the reference beam used to record a hologram is applied as a reading beam to reproduce the hologram. Aberration is thereby eliminated, making it possible to make the system compact. The term xe2x80x9cconjugate beamxe2x80x9d means a light beam that has the same wave front as the reference beam and propagates in the opposite direction.
In this method, however, the diffraction efficiency of the hologram must be compared with the threshold value and the system is complicated in structure. Furthermore, it is necessary to convert a light beam to an electric signal by the imaging device such as a CCD. Also is it necessary to convert the electric signal to a light beam by the spatial light modulator such as a liquid crystal display. Cause of errors, such as shot noise, inevitably increases.
To prevent a decrease in the diffraction efficiency of a hologram recorded in photo-refractive crystal, despite the light applied to the crystal to reproduce the hologram, it is proposed that two waves of different lengths be used to record a hologram. This method proposed has proved to be effective. (See U.S. Pat. No. 5,665,493 to Bai et al., Y. S. Bai and R. Kachru, Phys. Rev. Lett, 78, 2944, 1997.) In this method, crystals of Pr: LiNbO3, Pr: LiTaO3 and the like are used as the material of the recording medium. The method records a hologram in two steps. First, electrons at a low level are excited to an intermediate level, with blue light having a wavelength of about 450 nm. Second, infrared rays having a wavelength of about 850 nm are applied, thereby recording the hologram.
This method is, however, disadvantageous in some respects. First, two light sources are required. Second, the recording medium used has a lower sensitivity than one made of ordinary photo-refractive crystal. Further, the recording medium deteriorates over a long storage time, to the same extent as the conventional medium made of photo-refractive crystal.
The present invention has been made in view of the foregoing. The object of the invention is to provide a method and apparatus for recording a hologram on and reproducing a hologram from a hologram recording medium. The method and apparatus are characterized in that the decrease in the diffraction efficiency of the hologram, which occurs as the hologram is reproduced from the medium, is effectively suppressed, thereby lengthening the lifetime of the hologram.
A hologram recording/reproducing method according to this invention comprises the steps of: providing a hologram recording medium on which a hologram has been recorded by interference between an object beam and a reference beam; simultaneously applying a beam identical to the reference beam and a beam conjugate to the reference beam, as reading beams, to the hologram recording medium; forming a hologram by interference between a beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam; forming a hologram by interference between a beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam; and recording the holograms on the hologram recording medium, superposed one on the other, thereby reproducing the hologram.
In the hologram recording/reproducing method, the beam identical to the reference beam applied to record the hologram and the beam conjugate to the reference beam are simultaneously applied, as reading beams, to the hologram recording medium. As a result, the beam reproduced from the beam identical to the reference beam interferes with the beam conjugate to the reference beam, and the beam reproduced from the beam conjugate to the reference beam interferes with the beam identical to the reference beam.
The beam conjugate to the reference beam has the same wave front as the reference beam and propagates in the opposite direction to the reference beam. Hence, the hologram formed by interference between the beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam, and the hologram formed by interference between the beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam are identical to the original program formed on the hologram recording medium.
Therefore, with the hologram recording/reproducing method it is possible to record a new hologram identical to the original hologram on the hologram recording medium, while reproducing the original hologram. This helps suppress the decrease in the diffraction efficiency that occurs when the hologram is reproduced.
A hologram recording/reproducing apparatus according to the invention comprises: a recording medium holding section for holding a hologram recording medium on which a hologram has been recorded by interference between an object beam and a reference beam; and a hologram recording/reproducing section for simultaneously applying a beam identical to the reference beam and a beam conjugate to the reference beam, as reading beams, to the hologram recording medium, forming a hologram by interference between a beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam, forming a hologram by interference between a beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam; and recording the holograms on the hologram recording medium, superposed one on the other, thereby reproducing the hologram.
In the hologram recording/reproducing apparatus, the recording medium holding section holds the hologram recording medium on which a hologram has been recorded. The hologram recording/reproducing section simultaneously applies a beam identical to the reference beam and a beam conjugate to the reference beam, as reading beams, to the hologram recording medium. As a result, the beam reproduced from the beam identical to the reference beam and the beam reproduced from the beam conjugate to the reference beam interfere with each other, and the beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam interfere with each other.
The beam conjugate to the reference beam has the same wave front as the reference beam and propagates in the opposite direction to the reference beam. Hence, the hologram formed by interference between the beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam, and the hologram formed by interference between the beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam are identical to the original program formed on the hologram recording medium.
Therefore, with the hologram recording/reproducing apparatus it is possible to record a new hologram identical to the original hologram on the hologram recording medium, while reproducing the original hologram. This helps suppress the decrease in the diffraction efficiency that occurs when the hologram is reproduced.
In the hologram recording/reproducing method according to this invention, a beam identical to the reference beam and a beam conjugate to the reference beam are simultaneously applied, as reading beams, to a hologram recording medium on which a hologram has been recorded. A hologram is formed by interference between a beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam. Further, another hologram is formed by interference between a beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam. The holograms on the hologram recording medium is recorded and superposed one on the other, thereby reproducing the hologram. Therefore, the decrease in the diffraction efficiency of the hologram can be suppressed, thus lengthening the lifetime of the hologram.
In the hologram recording/reproducing apparatus according to the invention, the hologram recording/reproducing section simultaneously applies a beam identical to the reference beam and a beam conjugate to the reference beam, as reading beams, to the a hologram recording medium which is held by the recording medium holding means and on which a hologram has been recorded. A hologram is formed by interference between a beam reproduced from the beam identical to the reference beam and the beam conjugate to the reference beam. Another hologram is formed by interference between a beam reproduced from the beam conjugate to the reference beam and the beam identical to the reference beam. These holograms are recorded on the hologram recording medium and superposed one on the other, thereby reproducing the hologram. Hence, the decrease in the diffraction efficiency of the hologram can be suppressed, thus lengthening the lifetime of the hologram.